An Aluminum doped Zinc-Oxide (hereafter, “ZnAlO”) which is represented by a formula Zn1-xAlxO (0.01≦x≦0.05) where a part of zinc is replaced by aluminum, is known as n-type thermoelectric conversion material (patent references 1, 2, non-patent reference 1.)
ZnAlO exhibits a large electric conductivity (about 1000 S/cm) and a large Seebeck coefficient (100 to 200 μV/deg C in absolute value; Seebeck coefficient in a n-typed material is expressed by a minus value) in a wide temperature range (0 to 1000 deg C.), thus a power factor obtainable also is 5 to 10 times larger than those of iron-silicide series thermoelectric conversion materials.
ZnAlO does not include expensive rare earth elements and therefore has such features as low cost manufacturing, low toxicity against human body, and the power factor which is larger than other n-type thermoelectric conversion materials. On the other hand, thermal conductivity is extremely larger than those of other n-type thermoelectric materials. Because of that reason, there is a problem that a figure of merit Z cannot be enhanced large enough, in spite of the large power factor.
ZnAlO is obtained by sintering the power material at 1200 to 1400 degree C. Much efforts have been devoted in order to reduce the large thermal conductivity by improving manufacturing methods including: obtaining a sintered body of ZnAlO having crystal particle size of not greater than 40 μm from material of average particle size of around 300 μm (Patent reference 3), causing solid solution of lanthanum or nickel into ZnAlO (patent reference 4), causing crystalline orientation by mixing a material which generates an electrically conductive thermoelectric oxides having crystal anisotropy with ZnAlO (patent reference 5), replacing a part of Zn sites of ZnAlO by Fe (patent reference 6), manufacturing ZnAlO by the discharge plasma sintering (patent reference 7) and so on. The inventors of the present application reported that ZT=0.65 was obtained by introducing nanosized pores into a dense ZnAlO sintered body (patent reference 8, non patent reference 2).
By the way, oxide thermoelectric conversion materials are manufactured by the sintering method in general, ZnO series film materials may be manufactured by using film deposition methods such as sputtering method, vacuum deposition method, CVD method, laser ablation method (patent reference 9).    Patent reference 1: Japanese laid-open patent publication No. 62-132380,    Patent reference 2: Japanese laid-open patent publication No. 08-186293,    Patent reference 3: Japanese laid-open patent publication No. 2001-044520,    Patent reference 4: Japanese laid-open patent publication No. 2001-284661,    Patent reference 5: Japanese laid-open patent publication No. 2002-016297,    Patent reference 6: Japanese laid-open patent publication No. 2007-059491,    Patent reference 7: Japanese laid-open patent publication No. 2007-246294,    Patent reference 8: International Publication Pamphlet No. WO2005/091393A1,    Patent reference 9: Japanese laid-open patent Publication No. 2004-146586,    Non-Patent reference 1: M. Ohtaki et al., J. Appl. Phys., 79, 11816 (1996)    Non-Patent reference 2: M. Ohtaki et al., Proc. 25th Int. Conf. Thermoelectrics, pp. 276-279 (2006)